This invention pertains to electrochemical cells and, more particularly, is concerned with anode materials for such cells.
Electrochemical cells with lithium anodes provide high power density. Pure lithium metal metal melts at 181.degree. C., a temperature which can occur under short circuit conditions. Molten lithium can participate in uncontrolled exothermic chemical reactions with other cell components including the electrolyte and glass insulators. Accordingly, various lithium alloys have been investigated as alternatives to pure lithium metal anodes. Many such alloys of lithium are metallic solutions wherein the companion metal is soluble in lithium. Solution alloys include those of lithium with aluminum, boron, calcium, magnesium, mercury, lead and bismuth. Such solution alloys melt at temperatures well above that of pure lithium. For example, an alloy of fifty atomic percent aluminum and fifty atomic percent lithium melts at 718.degree. C., which is higher than either component metal alone. Solution alloy anodes have the disadvantage of causing voltage drops greater than that of an anode of pure lithium.
Non-solution lithium alloys have the advantage of providing the same voltage as pure lithium. In a non-solution lithium alloy, lithium is mixed with a companion metal having very sparing solubility in lithium and a higher melting point. This results in domains of pure lithium contained in a structural matrix of undissolved companion metal. At temperature above 181.degree. C., the domains of lithium will melt, but be contained by the structural matrix of undissolved companion metal which remains solid. This prevents the molten lithium from flowing and reacting with other cell components. A known companion metal for non-solution lithium alloy is boron, as reported by Larrick et al. Proc., 28 Power Sources Symp., Electrochemical Society Press, 1978, and Holmes, Proc. Symp. Lithium Batteries, Electrochemical Society Press, 1984. Lithium-boron alloy has a lower usable temperature point than desired.
It is desirable to provide alternate non-solution alloys for lithium electrochemical cells.